What does ifconfig -a show when it's not working? Is there an eth0? If it's not, I'd check the modules blacklists /etc/modprobe.d/blacklist*. I would have expected the recovery console to honour these though so moving on...

If you do have an interface, it could be some dodgy config inside network-manager. Recovery mode just uses some defaults IIRC so I'd open up the network settings (right click the applet, click edit connections) and check a few things:

Connect Automatically is checked

You have a MAC address

MTU is automatic

802.1x security is disabled

IPv4 is set to Automatic (DHCP)

Require IPv4 is checked

There should be no special routes

IPv6 is set to ignore

If ifconfig -a isn't showing an eth0 device, it could be something bizarrely hardware specific. What does lspci | grep net show? If nothing, do you know what the network chipset is?